TL;DR
In my case it’s totally the Pioneer AVR’s fault. When connecting the Odroid N2 directly to the TV everything works.
I have found a personal solution for a.m. issue.
When I switch on the LG TV my Pioneer AVR does not allow the CEC Power On command to pass through to the Odroid N2 when the AVR is in standby. This explains why I have to shortly switch inputs on the TV to make the Odroid N2 wake up after the TV has been switched on (because the AVR has been switched on by means of CEC by the TV).
My regular device chain is: Odroid N2 -> Pioneer AVR -> LG TV (HDMI2 - eARC)
My solution:
I have now connected the Odroid N2 directly to the LG TV and use eARC for all sound. Kodi is set to passthrough all audio over HDMI. This way the Odroid N2 receives the CEC Power On signal from the LG TV properly.
My revised device chain is now: Odroid N2 -> LG TV (HDMI1); LG TV (HDMI2 - eARC) -> Pioneer AVR
I have found just one audio format which doesn’t work: LPCM 5.1 - this is documented and according to LG US will be be fixed in an upcoming firmware update.